stop duplicating the broker plugins in the 'java release' binary

Details

Description

The 'java release' binary (which is effectively a horrible tar of the build/lib, build/bin, and build/etc directories) produced by running "ant release" currently contains a duplicate copy of the broker-plugins jars, increasing its size by around 5MB.

This is due to the way they are collected into a sub-directory for later use by the release-bin target (following changes in QPID-4456). The directory should be moved out of build/lib in order to prevent this duplication in the tar file.